This invention relates to a device for sewing an overedge stitch on a material. More particularly, this invention relates to sewing a flexible overedge seam without experiencing high thread breakage.
A goal in the sewing industry is to produce a soft and flexible overedge stitch with light tensions applied on the sewing threads during the sewing operation. The sewing industry requires the threads to produce a soft appearing overseam stitch and to accommodate the high tensions experienced in the overseaming operation. High tensions on the sewing threads cause thread breakage and increase operation downtime for thread breakage repair. However, the industry has sought to reduce needle thread tension experienced in the overseaming operation because knit manufactures sometimes require that certain weak knitting yarns, such as texturized polyester, be used for sewing threads. In addition, light needle thread tensions reduce or eliminate needle deflection caused by excessive needle thread tensions. The industry also seeks a soft, flexible chain because it eliminates the need for mechanical aids to provide flexibility in a latch tacking operation. Also, a flexible chain moves easily into a chain trimmer without hang-up, is less likely to unravel, and is more durable.
A flexible overedge seam sewn into a material should have a tight needle thread and relaxed upper and lower looper threads. A flexible chain is produced by the upper and lower looper threads setting on the edge of the stitch tongue before the needle thread sets. This results in the needle thread wrapping around the stitch tongue after the looper threads are set. When not sewing in a material, the needle thread length in the overedge stitch is approximately the same as the length of the upper and lower looper threads when the chain is stretched out. However, when sewing an overseam into a material, the needle thread sets slightly before the lower looper thread causing the needle thread to become pulled up by the needle thread cam and remain tight in the seam.
Three prior art devices for sewing an overedge stitch or an overseam are the Union Special Class 39500, HV 200 series and Class EV-900 sewing machines, and the Juki MO-2500N and MO-2400N series sewing machines. The Union Special 39500, which was the first overedge stitch machine, employs a curved sewing needle and a lower looper that travels in a straight direction. The timing of the lower looper with respect to the curved sewing needle is advanced 30.degree. and the needle thread is in partial contact with the needle thread cam 75% of the time during the needle stroke. However, the Union Special 39500 sewing needle is expensive because it is very costly to manufacture a curved needle in comparison to a straight needle.
The Union Special HV 200 series overedge sewing machine, which was developed after the Union Special 39500, employs a straight sewing needle, and a lower looper with a 20.degree. advanced timing with respect to the needle. The lower looper travels in a straight path and the needle thread cam contacts the needle thread approximately 80% of the time during the needle stroke. While the straight needle eliminates the expense of machining a curved needle, the Union Special HV 200 series machine has a tendency to produce a malform stitch because of the 20.degree. advanced timing.
More recently, the Juki MO-2500N and MO-2400N series sewing machines, as well as the Union Special Class EV-900 sewing machines, have been used for performing overseaming operations. These machines contain a straight sewing needle and a lower looper with a synchronized timing with respect to the sewing needle. The lower looper travels in a curved path and the needle thread contacts the needle thread cam substantially 100% of the time during the needle stroke. This design suffers from high needle thread breakage because of the high friction caused by the synchronized timing and the 100% contact of the needle thread with the needle thread cam.